Wireless communication systems following Universal Mobile Telecommunications Systems (UMTS) technology, were developed as part of Third Generation (3G) Radio Systems, and is maintained by the Third Generation Partnership Project (3GPP). A mobile radio communication system, such as a UMTS type system, includes a mobile radio communication network communicating with mobile terminals or UEs (User Equipments) and with external networks. The UMTS network architecture includes a Core Network (CN) interconnected with a UMTS Terrestrial Radio Access Network (UTRAN) via an Iu interface. The UTRAN is configured to provide wireless telecommunication services to users through mobile radio terminals, referred to as user equipments (UEs) in the 3GPP standard, via a Uu radio interface. A commonly employed air interface defined in the UMTS standard is wideband code division multiple access (W-CDMA). The UTRAN has one or more radio network controllers (RNCs) and base stations, referred to as Node Bs by 3GPP, which collectively provide for the geographic coverage for wireless communications with UEs. Uplink (UL) communications refer to transmissions from UE to Node B, and downlink (DL) communications refer to transmissions from Node B to UE. One or more Node Bs are connected to each RNC via an Iub interface; RNCs within a UTRAN communicate via an Iur interface.
Mobile networks with High Speed Packet Access (HSPA) include High Speed Downlink Packet Access (HSDPA) and High Speed Uplink Packet Access (HSUPA) or Enhanced Uplink (EUL). The physical channels of the older Release 99 (R99) and newer EUL are shown in FIG. 1 on the right and left sides, respectively. Enhanced Uplink (EUL) introduces two new code-multiplexed uplink physical channels compared to R99. One is an enhanced data channel, E-DCH Dedicated Physical Data Channel (E-DPDCH), and the other is an enhanced control channel, E-DCH Dedicated Physical Control Channel (E-DPCCH). In EUL, the Dedicated Physical Control Channel (DPCCH) is used in the similar way as in R99 carrying pilot, power control, and Inner Loop Power Control (ILPC) information. The transport format of EUL is designated as E-DCH Transport Format Combination (E-TFC). A standard E-TFC table is set forth in 3GPP specification 25.321. In the E-TFC table, a transmit power gain factor named βed is used to indicate the enhanced data channel E-DPDCH amplitude for each E-TFC in the table, and a transmit power gain factor named βec is used to indicate the amplitude of E-DPCCH. In this table, the βec value is fixed for all E-TFCs, and the βed value is unique for each E-TFC value. In R99, the transmit power level of the DPDCH is indicated by βd for each transport format respectively. In both EUL and R99, the same parameter, βc, is used to indicate the DPCCH transmit power level. The power offsets for data and other control channels relative to the allowed DPCCH transmit power level can be calculated and are shown in FIG. 1 for both EUL and R99 systems, where N is the number of E-DPDCHs or DPDCHs. A predetermined small minimum transmit power level of E-DPDCH is specified using βed, min in 3GPP specification 25.214 (R6), and the minimum data power offset based on βed, min is also shown in FIG. 1. In the uplink, DPCCH is used as a power reference with the power offset of all the other physical channels being defined relative to the DPCCH power.
EUL increases the uplink data rate and capacity compared to that in R99. In addition, the transmission time interval (TTI) length in EUL Release 6 (R6) is expanded to include a 2 ms TTI in addition to a 10 ms TTI. The introduction of the shorter 2 ms TTI in EUL unfortunately increases the risk of unduly limiting the UE transmission power on the uplink data channel E-DPDCH. For instance, a UE needs about 7 dB higher transmit power to send a voice packet over a 2 ms TTI as compared to sending it over a 10 ms TTI so that the packet is received with the same quality at the receiver side for a single transmission attempt. Since the use of multiple transmission attempts may not always be feasible or desirable, the coverage aspect of EUL systems with 2 ms TTIs should be addressed. The uplink coverage includes for example an area where a desired the Quality of Service (QoS) for an uplink radio transmission from a UE in that area to the network for a predetermined traffic load can be provided.
The R6 EUL specification stipulates that when the UE is “power-limited,” i.e., the required total uplink transmit power for a UE's desired uplink communication exceeds the UE's maximum transmit power, the power level for the uplink transmission on the E-DPDCH is scaled down to a small predetermined minimum E-DCH power ratio. In contrast, the DPCCH is not scaled down in order to protect the DPCCH quality. This reduction of the E-DPDCH power level leads to an increase of transmission failures which then triggers the outer loop power control (OLPC) functionality to increase the DPCCH signal-to-interference (SIR) target used in the OLPC, leaving even less power for the E-DCH data channel. Another important issue is that the anti-windup mechanism of the Outer Loop Power Control (OLPC) will not prevent the SIR target increase. Although anti-windup prevents further increase of the DPCCH SIR target value if the current received DPCCH SIR can not reach the stipulated DPCCH SIR target, with the power scaling strategy in 3GPP release R6, the DPCCH SIR can still reach the SIR target even in a serious power limitation situation by scaling down E-DCH power. As a result, in the process of limiting uplink data channel power in release R6, the SIR target rapidly reaches the maximum SIR target limit.
So in summary, the justification for down-scaling the data channel power level to a very low predefined power offset in R6 was to protect the DPCCH. But this protection mechanism can produce too high of a DPCCH SIR and too low of a available E-DCH power. Ultimately, this low available E-DCH power results in small EUL coverage. One way to address this is problem is to use a configurable transmit power gain factor βed, min, which can avoid excessive down-scaling of the data channel E-DCH power by setting a minimum power level for the E-DCH. The configurable βed, min permits a better trade-off of the power allocation between the E-DCH and the DPCCH control channel during UE power limitation, which in turn improves the EUL coverage by 4-5 dB. FIG. 2 illustrates that more power is allocated for E-DCH during power limitation with a configurable βed, min compared to E-DCH power down-scaling in 3GPP R6.
Two other methods which could be used to overcome the UE power limitation problems and improve EUL coverage include autonomous retransmission and “improved layer 2” (L2). Autonomous retransmission is also called TTI bundling, which means that UE autonomously transmits one packet multiple times in consecutive TTIs according to the UE power limitation level in order to achieve a desired E-DCH reliability over a large coverage area than would be possible without such autonomous retransmission. Improved L2 also improves EUL coverage by separating data into various smaller media access control (MAC) packet data units (PDUs) so that smaller E-TFCs can be used during UE power limitation. For instance, a Voice over IP (VoIP) user generates a voice packet every 20 ms. Smaller packets can achieve more transmit power per bit during UE power limitation, thereby improving the VoIP quality for this UE.
Given that a configurable βed, min, which determines the lower E-DPDCH transmit power level, is going to be accepted in 3GPP R8, an important issue is how to determine the proper βed, min value and/or range in order to achieve as large coverage gain. The proper βed, min value and/or range assures that the Block Error Probability (BLEP) for the E-DCH after a maximum allowable number of transmission attempts by the UE on the E-DCH achieves the minimum service quality requirement for a desired uplink coverage area. The proper βed, min value and/or range should also be compatible with other techniques to improve EUL coverage such as autonomous retransmission and improved L2.